Some fishing boats in the US label each fish and put it in a data base, so the eventual purchaser can check what it was and where it was caught.

It seems like a good idea (since dead fish are not entitled to privacy rights), but I don't think it will entirely stop the fraud of mislabeling. It won't be impossible for fishermen to put lies into the system, or for supermarkets and restaurants to lie about what the system said about the fish they are selling.

A portable, quick DNA testing probe would make it easy to hold the whole system to account. They won't dare lie if they expect to be caught and fined.